1. Field of the Invention
This invention relates to a semiconductor device and its manufacturing method, especially of a MOS transistor, or the like, having micro defects for gettering.
2. Description of the Prior Art
In a manufacturing process of semiconductor devices, protection of semiconductor substrates from contamination by metal impurities is one of most important techniques for manufacturing processes because such contamination adversely affects the electric characteristics of semiconductor devices. Especially in manufacturing processes of MOS device protection of semiconductor substrate from contamination is quite important. Therefore, efforts are made to clean manufacturing processes by cleaning and purifying manufacturing machines, apparatuses or materials, or by developing cleaning techniques of semiconductor substrate. In addition to these methods, there is a method called gettering to suppress behaviors of metal impurities once diffused into semiconductor substrate.
There are various methods of gettering. One of most effective methods is to make oxygen-induced micro defects (BMD: bulk micro defects) within a semiconductor substrate to use them as capture site of metal impurities. However, it is undesirable that these micro defects exist in a device active layer nearer to a surface of the semiconductor substrate, because metal impurities captured into the micro defects will significantly degrade the electric characteristics of the device. Therefore, when metal impurities are removed from the device active layer by capturing them into the micro defects, it is important not to make micro defects in the device active layer. A non-defective layer with no or very few micro defects is called denuded zone (DZ). The denuded zone is required to be deeper than the device active layer.
In order to make a denuded zone and micro defects, interstitial oxygen has to be present in a semiconductor wafer of silicon, for example. In this respect, the following two methods are generally used to make the denuded zone. In case of a silicon semiconductor wafer, there are the method of additionally stacking a silicon layer without oxygen in a region in which micro defects should be made epitaxial substrate, and the method of annealing the silicon substrate to externally diffuse oxygen from the surface layer zone of the substrate so that the interstitial oxygen concentration decreases in the surface layer zone of the silicon substrate and micro defects are not produced in the surface layer zone. Both these methods control the distribution of micro defects by controlling the distribution of oxygen concentration along the depth. Therefore, the density of micro defects suddenly increases in the transitional region between the denuded zone and the oxygen-educed zone. However, distribution of the micro defects density from the transitional region toward a deeper direction is uniform.
On the other hand, various methods for device isolation semiconductor elements have been developed recently for higher and higher integration of semiconductor element. STI (shallow trench isolation) is one of such isolation techniques for semiconductor devices. When a trench is made in a semiconductor substrate of silicon, or the like, for STI, a stress is liable to be applied to the semiconductor substrate. Moreover, as semiconductor wafers are getting bigger, they are liable to deflect due to its own weight during annealing. It is estimated that the stress applied to the STI zone and deflection of the wafer result in applying a considerable stress to the semiconductor wafer surface layer.
When the stress applied to the semiconductor wafer exceeds a critical value, plastic deformation occurs in the semiconductor wafer. Since plastic deformation accompanies dislocation and its growth, it seriously affects the electric characteristics of the semiconductor device. On the other hand, it is effective for a semiconductor wafer to contain high concentrated interstitial oxygen for a higher mechanical strength thereof. Therefore, it is required to be well-balanced with gettering (creation of micro defects and formation of a denuded zone). Heretofore, however, attention was paid only to control of the entire density of micro defects, and no proposal has been heard of on regulation of mechanical strength of semiconductor wafers by control of the size and density of micro defects in the depth direction of the semiconductor wafer.